One emerging area of networked communications is the provisioning of sensor networks. Sensor networks enable the gathering of information throughout an environment, and have applications, for example, in monitoring areas (such as battlefields, buildings, or other geographical areas), industrial processes (such as machine conditions or fluid flow rates), transportation systems (such as traffic flow or traffic density), and patient health (such as any number of physiological conditions of a given patient). Many other applications exist as well.
Generally, sensor networks consist of a set of network nodes, or sensor nodes, and a base station, or controller, with which the sensor nodes may communicate. The sensor nodes may be configured to monitor environment conditions in accordance with, for example, any of the applications mentioned herein. For example, in an industrial processes application, the sensor node may be configured to monitor an environment condition such as fluid flow rate (at some point in an industrial process). As another example, in a patient health application, the sensor node may be configured to monitor an environment condition such as a physiological condition (of a body).
A sensor network may assume any one or more of a variety of network topologies, examples of which may include star, ring, bus, tree, fully connected, cluster-based, mesh, and/or any other network topology now known or later developed. Further, the topology of a given sensor network may change, perhaps frequently.
Sensor networks may be wired or wireless. Among other benefits, wireless sensor networks may, generally, be deployed with less cost, effort, and resources than wired sensor networks. Also, wireless sensor networks are generally more flexible and robust, and may therefore be put to use in more applications.
In a wireless sensor network, the sensor nodes and base station typically communicate over an air interface according to an established communication protocol, or an agreed method of managing communications in the wireless communication network. Examples of common communication protocols include CDMA (e.g., EIA/TIA/IS-2000 Rel. 0, Rel. A, or other versions thereof (“IS-2000”), EIA/TIA/IS-856 Rel. 0, Rev. A, or other versions thereof (“IS-856”), etc.), iDEN, TDMA, AMPS, GSM, GPRS, UMTS, EDGE, WiMAX (e.g., IEEE 802.16), LTE, microwave, satellite, MMDS, Wi-Fi (e.g., IEEE 802.11), Bluetooth, infrared, or any other communication protocol now known or later developed. A communication protocol may specify, among other things, data formats, address formats, switching and routing processes, and flow control processes for network nodes operating under the communication protocol. A communication protocol used by a given sensor network may incorporate one of the common communication protocols delineated above, or may incorporate a communication protocol that has been developed, at least in part, for sensor networks specifically.
Typical communication protocols employed by sensor networks may be broadly categorized into contention-based protocols and polling-based protocols. Contention-based protocols generally allow for sensor nodes to communicate in a given sensor network without pre-coordination. For example, a contention-based protocol may define the events that must occur when two or more sensor nodes attempt to simultaneously communicate on a given air interface, and establish rules by which sensor nodes are given opportunities to communicate on the given air interface.
On the other hand, polling-based protocols generally involve a central network node, such as a base station, which periodically interrogates sensor nodes in a round-robin, recurring, or other sequence to determine whether the sensor nodes have data to send to the base station. Polling-based protocols therefore require some degree of pre-coordination. The interrogation of a sensor node by the base station may take the form of a paging message, sent over a paging channel of the air interface, that serves to notify a sensor node that it is being paged.
The paging channel may be divided into a number of timeslots of equal length in which the base station may poll, or page, the sensor nodes. Each sensor node may be arranged to wake up and check the paging channel for a page with a given frequency. When paged, the sensor node may respond, or otherwise react, in a manner specified by the particular polling protocol under which the sensor network is operating. As one example, when paged, the sensor node may transmit certain data to the base station.
Generally speaking, the more often a sensor node is polled and transmits data, the more quickly the sensor node will lose energy. Therefore, in this respect, it is desirable for a sensor node to check for pages and transmit data relatively infrequently. However, there are competing considerations that make it desirable for a sensor node to check for pages and transmit data relatively frequently. For example, it may be desirable for the sensor node to transmit data to the base station relatively frequently if the data collected by the sensor node (e.g., as a result of monitoring an environment condition) represents information that is time sensitive or otherwise important. If the sensor node transmits data relatively frequently, the sensor node may transmit data with relatively low latency, where latency is the amount of time between when the sensor node is ready to transmit data and when the sensor node actually does transmit the data. In this respect, the lower the latency with which a sensor node transmits data, the better the performance of the sensor node—and therefore the sensor network—will be.
Attempts to improve the energy-efficiency of sensor networks have taken many forms, including attempts to improve sensor-transceiver design, sensor-network architecture, and energy-scavenging techniques. However, wireless communication is also an important factor in energy consumption in sensor networks, and therefore the development of efficient communication protocols, or methods of managing communications, for sensor networks is of particular interest.